In some sort of functional materials, movement of solvent molecules in the materials may govern performances of the materials. In design and development of this sort of materials, local measurement of mobility of the solvent molecules is understood as an important technical issue. This sort of functional material can be exemplified by solid polymer electrolyte film used for fuel cells.
In fuel cells using the solid polymer electrolyte film, power generation characteristics and efficiency strongly depend on ion conductivity of the polymer film. To keep the power generation characteristics at a high level, it is necessary to keep the ion conductivity of the polymer film at a high level. The ion conductivity of the film is induced by movement of hydrogen ion in the film, so that mobility of the hydrogen ion governs the ion conductivity of the polymer film. The hydrogen ion does not move alone in the film, but moves in the film together with water molecules disposed therearound, while canceling the electric charge by the polar water molecules, so as to protect and allow the hydrogen ion to stably exist in the film. The water molecule moves together with hydrogen ion is called “electro-osmotic water (water taranport by electro-osmotic drag)”, and plays an important role in keeping the ion conductivity of the polymer film at a high level.
In relation to this transportation mechanism in the polymer film, it is known that the ion conductivity in the polymer film is determined by the amount of water molecules contained in the film (water content of the film) and readiness of movement of water molecules (mobility of water molecules) in the film. More specifically, the amount of water M moving in the solid polymer electrolyte film is expressed, using water content m in the film and mobility v of water molecules, asM=mv
A technique of locally measuring m has already been proposed by the present inventors (Japanese Patent Application No. 2004-265535).
On the other hand, a technique of measuring “readiness of movement of water molecules (mobility of water molecules) in the film” has not been proposed, with a partial exception, despite its importance to the ion conductivity of the polymer material. In particular, measurement of “mobility of water molecules” is essential in order solve a problem that the amount of power generation sharply decreases in the process of power generation by fuel cells. At present, decrease in the amount power generation is supposedly ascribable to lowering in the ion conductivity or deterioration of catalyst in the polymer film, although definitive evidence remains unknown, demanding a technique of monitoring properties of the film during power generation. In this process, only insufficient information is available simply by a technique of measuring the water content of the film, wherein “causes for degradation of the ion conductivity” can be elucidated by measuring the “mobility of water molecules in the film” at the same time. A guideline for the countermeasure can be obtained only after the elucidating the causes for the degradation.
Other than those described in the above exemplified by the solid polymer electrolyte film used for fuel cells, there are large needs for techniques of measuring mobility of molecules of solvent such as water in solid matrix and gel, wherein these technique of measurement may be key technology for the material development.
There has been developed several techniques of measuring mobility of solvent molecules in solid.
Techniques of measuring “mobility of water molecules” in polymer film includes (i) a technique of measuring “mobility through a film applied with liquid under pressure, based on the amount of permeation” (Non-Patent Document 1). This method is, however, disadvantageous in that “the mobility through the polymer film is not measurable under varied conditions of moistening”, because the both surfaces of the film is immersed in water. For the process of power generation in fuel cells, it is necessary to understand not only the situation where the polymer film is immersed in water, but also “mobility of water through the polymer film under different conditions of moistening” which may vary depending on conditions of moistening, but the method cannot alter the conditions of moistening. Moreover, the method is not considered as enabling quick local measurement.
There are also known conventional methods of measuring “mobility of water molecules”, such as a method of (ii) “measuring mobility of water molecules in terms of self-diffusion coefficient” based on the nuclear magnetic resonance (NMR) method; and a method of (iii) “measuring mobility of water molecules as being expressed by an image of distribution of self-diffusion coefficient” based on the magnetic resonance imaging (MRI) method. The method (ii) in the above is a publicly-known method described in Non-Patent Document 2, by which the entire portion of a sample is measured, so as to calculate a mean mobility of water molecules.
The method of (iii) “measuring mobility of water molecules as being expressed by an image of distribution of self-diffusion coefficient” in the above is a technique based on combination of MRI and the above-described (ii) so as to provide imaging of the distribution, and is publicly known, similarly to the technique (ii) in the above, as being called “diffusion imaging” (Non-Patent Document 3), or “MR image emphasizing diffusion of water molecules” (Non-Patent Document 4).
[Non-Patent Document 1] “Ko-bunshi to Mizu (Polyer and Water)”, edited by The Society of Polymer Science, Japan, Chapter 3
[Non-Patent Document 2] E. O. Stejskal and J. E. Tanner, “Spin diffusion measurements: Spin Echoes in the Presence of a Time-Dependent Field Gradient”, Journal of chemical physics, vol. 42, No. 1, 1965, pp. 288-292
[Non-Patent Document 3] NMR Imaging, Katsumi KOSE, Kyoritsu Shuppan Co., Ltd., (2004), p. 176
[Non-Patent Document 4] The 13th JAMIT Seminar (October, 1992, Tokyo), Med. Imag. Tech., Vol. 11, No. 1, 1993, p. 12-21